The present invention relates to damping cylinders and gas springs.
Damping cylinders and gas springs are widely used in a variety of applications. Perhaps the most visible application is the counterbalance support of liftgates, trunk lids, and other automotive doors. Other applications include furniture, such as height-adjustable chairs and work surfaces. One such furniture application is illustrated in co-pending application Ser. No. 08/584,266 filed Jan. 11, 1996 by Long.
The construction of such damping cylinders and gas springs are well known. Exemplary constructions are illustrated in U.S. Pat. No. 4,045,008 issued Aug. 30, 1977 to Bauer; U.S. Pat. No. 4,156,523 issued May 29, 1979 to Bauer; U.S. Pat. No. 5,215,291 issued Jun. 1, 1993 to Bauer et al; and U.S. Pat. No. 5,615,867 issued Apr. 1, 1997 to Bauer. Such a cylinder includes a tube at least partially filled with a liquid. A rod having a piston on one end is mounted for reciprocable movement within the cylinder so that the piston travels within the fluid. The piston defines one or more ports through which the fluid passes as the piston moves to provide the damping effect. If the cylinder is a gas spring, a pressurized gas is included within the cylinder to provide an output force or a spring force.
The rate at which the rod is capable of moving with respect to the cylinder depends on a variety of factors, including the size and number of the ports through the piston and the viscosity of the liquid within the tube. Once these parameters are selected and fixed for a particular cylinder, the rate at which the rod moves within the cylinder is largely a function of the axial load on the rod. For fixed loads such as liftgates, the rate will be relatively constant. However, with variable loads, the rate increases with increasing loads. This has made damping cylinders and gas springs undesirable for applications wherein the load on the rod varies significantly. If the rod bears an unusually high load, the assembly supported by the rod may move too quickly for convenient or even safe operation.